Bound state of crystal electric field excitation and a phonon in CePd₂Al₂

Petr Doležal¹, Petr Cejpek¹, Satoshi Tsutsui², Koji Kaneko³, Dominik Legut¹, Zuzana Mičková¹, Kristína Bartha⁴, Pavel Javorský¹

¹Department of Condensed Matter Physics, Charles University, Prague 121 16, The Czech Republic
²Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, Hyogo 679-5198, Japan
³Materials Sciences Research Center, JAEA, Tokai, Ibaraki 319-1195, Japan
⁴Department of Physics of Materials, Charles University, Prague 121 16, The Czech Republic

The existence of a bound state (vibron state) between 4f electrons and phonons is closely associated with CeAl₂ [1] compound. The magnetic excitations observed by neutron scattering were not possible to be described by standard crystal electric field theory using cubic symmetry for Ce surrounding. Also local static distortion on Ce site was excluded. The observed magnetic excitation was finally described by the Thalmeier-Fulde model of vibron state [2]. This model explains the observed magnetic excitations and also gives a prediction which phonon mode should be involved in the bound state (in case of CeAl₂ it is G₁₅). This phonon mode is then influenced by the vibron state – the phonon mode softens and broadens [1].

For a long time CeAl₂ was a unique compound in which this enhanced interaction was observed. The increasing interest in recent time is motivated by new tetragonal materials which exhibit similar behaviour of magnetic excitations: CePd₂Al₂ [3], CeCuAl₂ [4] and CeAuAl₃ [5]. But the study of phonon dispersion curves and the influence of the vibron state is still missing.

Our interest is focused on CePd₂Al₂ and non-magnetic analogue LaPd₂Al₂ which crystalize in tetragonal CeBe₂Ge₂ structure type and undergo a structural phase transition decreasing their symmetry in the basal plane. The resulting structure can be then described within the orthorhombic Cmme space group. We succeeded with preparation of single crystalline (Ce,La)Pd₂Al₂ samples and investigated their phonon dispersion curves, mainly the temperature dependence of A_1g phonon mode, see Fig. 1. The results will be discussed in terms of enhanced magneto-elastic interaction which is responsible for formation of the vibron state.

[1] P. Thalmeier, J. Phys. C Solid State Phys. 17, 4153-4177 (1984)

[2] M. Loewenhaupt et al., Phys. B 86-88, 187-188 (1977)

[3] L. C. Chapon et al., Phys. B 378-380, 819-820 (2006)

[4] D. T. Adroja et al., Phys. Rev. Lett. 108, 216402 (2012)

[5] P. Cermak et al., PNAS 14, 6695-6700 (2019)

This work was supported by the Czech Science Foundation under Grant no. 17-04925J and also within the project NanoCent, project no. CZ.02.1.01/0.0/0.0/15 003/0000485.